Modified novolak phenolic resin, making method, and resist composition

ABSTRACT

A modified novolak phenolic resin is obtained by reacting a novolak phenolic resin containing at least 50 wt % of p-cresol with a crosslinker. This method increases the molecular weight of the existing novolak phenolic resin containing at least 50 wt % of p-cresol to such a level that the resulting modified novolak phenolic resin has heat resistance enough for the photoresist application.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application Nos. 2011-233901 and 2012-176987 filed in Japan onOct. 25, 2011 and Aug. 9, 2012, respectively, the entire contents ofwhich are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a modified novolak phenolic resin obtained byreacting a novolak phenolic resin resulting from a phenol containing atleast 50% by weight of p-cresol with a crosslinker, a method ofpreparing the resin, and a resist composition comprising the resin.

BACKGROUND ART

Novolak phenolic resins are thermoplastic resins. Various means areknown for increasing the weight average molecular weight of novolakphenolic resins. For example, JP-A 2005-306987 describes a two-stagereaction method designed to increase the weight average molecular weightwhile reducing phenol dimer. JP-A 2004-292791 discloses a method ofpreparing triazine-modified novolak phenolic resin designed to increasethe weight average molecular weight while reducing unreacted phenol.These methods are not regarded versatile since they are limitative inthat polymerization in the presence of phosphoric acid is essential inthe first stage of synthesizing novolak phenolic resin. In addition, thesecond stage of reaction needs pH control and addition of aldehyde forthe purpose of reducing the phenol dimer or unreacted phenol. Themethods are more limitative in this sense too. They are not regarded assimple means for increasing the molecular weight of novolak phenolicresins. Also JP-A 2000-309617 discloses a condensates of phenolderivative, aldehyde and triazine derivative as a phenol-aminocondensation resin. This method is also limitative in thatpolymerization is carried out while restricting a molar ratio of amethylol group content to a methylene bond content in a certain rangeand effecting reaction in the co-presence of a triazine derivative andaldehyde. This is not regarded simple or brief in increasing themolecular weight of novolak phenolic resins.

On the other hand, resol phenolic resins are thermosetting resins. JP-AH03-243613 discloses a method of preparing a phenol-melamineco-condensation resin. However, this method is applicable to the novolakphenolic resins with difficulty because the synthesis of a phenolicresin as the starting resin is different between the novolak type andthe resol type.

CITATION LIST

Patent Document 1: JP-A 2005-306987

Patent Document 2: JP-A 2004-292791

Patent Document 3: JP-A 2000-309617

Patent Document 4: JP-A H03-243613

SUMMARY OF INVENTION

An object of the invention is to provide a modified novolak phenolicresin. That is, a novolak phenolic resin polymerized by a well-knownprocedure using a phenol containing at least 50% by weight of p-cresolis simply and briefly modified through versatile means into a modifiednovolak phenolic resin having a higher molecular weight and heatresistance enough for use as photoresist material. Another object is toprovide a method of preparing the modified novolak phenolic resin, and aresist composition using the modified novolak phenolic resin.

The inventors have found that when a novolak phenolic resin obtainedusing a phenol containing at least 50% by weight of p-cresol is modifiedby reacting it with a crosslinker in the presence of an acidic catalyst,the modified novolak phenolic resin exhibits sufficient thermalproperties and is useful as the phenolic resin for photoresist use.

Seeking for the means for converting a novolak phenolic resin obtainedusing a phenol containing at least 50% by weight of p-cresol to a highermolecular weight resin, the inventors have arrived at the reaction ofthe novolak phenolic resin with a crosslinker in the presence of anacidic catalyst, and have found that the resultant modified novolakphenolic resin exhibits significant properties.

In one aspect, the invention provides a modified novolak phenolic resinobtained by reacting (A) a novolak phenolic resin obtained fromcondensation of a phenol containing at least 50% by weight of p-cresoland an aldehyde, or (B) a mixture of (b-1) at least 50% by weight of ap-cresol novolak resin obtained from condensation of p-cresol and analdehyde and (b-2) the balance of another novolak phenolic resinobtained from condensation of a phenol other than p-cresol and analdehyde with a crosslinker.

In a preferred embodiment, the phenol from which the novolak phenolicresin (A) is obtained consists of 50 to 80% by weight of p-cresol andthe balance of another phenol.

In a preferred embodiment, the mixture (B) consists of 50 to 80% byweight of the p-cresol novolak resin (b-1) and the balance of the othernovolak phenolic resin (b-2).

In a preferred embodiment, the novolak phenolic resin (A), the p-cresolnovolak resin (b-1), and the other novolak phenolic resin (b-2) eachhave a weight average molecular weight in the range of 1,500 to 10,000.

In a preferred embodiment, the crosslinker is at least one memberselected from the group consisting of an amino condensate modified withformalin or formalin-alcohol, a phenol compound having on average atleast two methylol or alkoxymethylol groups in a molecule, and an epoxycompound having on average at least two epoxy groups in a molecule.Typically, the crosslinker is a modified melamine condensate or modifiedurea condensate.

In another aspect, the invention provides a method of preparing amodified novolak phenolic resin, comprising the steps of mixing (A) anovolak phenolic resin obtained from condensation of a phenol containingat least 50% by weight of p-cresol and an aldehyde, or (B) a mixture of(b-1) at least 50% by weight of a p-cresol novolak resin obtained fromcondensation of p-cresol and an aldehyde and (b-2) the balance ofanother novolak phenolic resin obtained from condensation of a phenolother than p-cresol and an aldehyde with a crosslinker, simultaneouslyor subsequently adding an acidic catalyst thereto, and effectingreaction.

In a preferred embodiment, the phenol from which the novolak phenolicresin (A) is obtained consists of 50 to 80% by weight of p-cresol andthe balance of another phenol.

In a preferred embodiment, the mixture (B) consists of 50 to 80% byweight of the p-cresol novolak resin (b-1) and the balance of the othernovolak phenolic resin (b-2).

In a preferred embodiment, the novolak phenolic resin (A), the p-cresolnovolak resin (b-1), and the other novolak phenolic resin (b-2) eachhave a weight average molecular weight in the range of 1,500 to 10,000.

In a preferred embodiment, the crosslinker is at least one memberselected from the group consisting of an amino condensate modified withformalin or formalin-alcohol, a phenol compound having on average atleast two methylol or alkoxymethylol groups in a molecule, and an epoxycompound having on average at least two epoxy groups in a molecule.Typically, the crosslinker is a modified melamine condensate or modifiedurea condensate.

In a preferred embodiment, the acidic catalyst is at least one acidselected from the group consisting of hydrochloric acid, sulfuric acid,boric acid, oxalic acid, acetic acid, benzenesulfonic acid,p-toluenesulfonic acid, xylenesulfonic acid, p-phenolsulfonic acid,methanesulfonic acid, and ethanesulfonic acid. Typically, the acidiccatalyst is an organic sulfonic acid.

In a preferred embodiment, the steps of simultaneously or subsequentlyadding an acidic catalyst and effecting reaction include holding at atemperature of at least 10° C.

In a further aspect, the invention provides a resist compositioncomprising the modified novolak phenolic resin defined above as a baseresin. Typically, the resist composition is a positive workingcomposition. The resist io composition may further comprise a1,2-naphthoquinonediazidosulfonic acid ester. Preferably, the1,2-naphthoquinonediazidosulfonic acid ester is an ester of some or allmodified phenolic hydroxyl groups on the modified novolak phenolic resindefined above with 1,2-naphthoquinonediazidosulfonic acid.

ADVANTAGEOUS EFFECTS OF INVENTION

The invention makes it easy to increase the molecular weight of anexisting novolak phenolic resin obtained using a phenol containing atleast 50% by weight of p-cresol. The resulting modified novolak phenolicresin has sufficient heat resistance for use in the photoresistapplication.

DESCRIPTION OF PREFERRED EMBODIMENTS

The modified novolak phenolic resin (I) of the invention is obtainedfrom a starting novolak phenolic resin (II) which may be either (A) anovolak phenolic resin obtained from condensation of a phenol containingat least 50% by weight of p-cresol and an aldehyde, or (B) a mixture of(b-1) at least 50% by weight of a p-cresol novolak resin obtained fromcondensation of p-cresol and an aldehyde and (b-2) the balance ofanother novolak phenolic resin obtained from condensation of a phenolother than p-cresol and an aldehyde. The novolak phenolic resin (A) maybe a copolymer. The p-cresol novolak resin (b-1) is a polymer. The othernovolak phenolic resin (b-2) may be a polymer, and is also referred toas p-cresol-free resin. These novolak phenolic resins may be prepared byany well-known methods, specifically by using a phenol(s) and analdehyde(s) as reactants, and reacting them in the presence of awell-known acidic catalyst, optionally in a reaction-mediating solvent.

In the novolak phenolic resin (A), p-cresol is essential as the phenolreactant and optionally, substituted or unsubstituted phenols may beused as the phenol reactant other than p-cresol. Suitable phenols whichcan be used herein include m-cresol, o-cresol, phenol, 2-allylphenol;xylenols such as 2,3-xylenol, 2,5-xylenol, 3,5-xylenol, and 3,4-xylenol;alkylphenols such as m-ethylphenol, p-ethylphenol, o-ethylphenol,2,3,5-trimethylphenol, 2,3,5-triethylphenol, 4-tert-butylphenol,3-tert-butylphenol, 2-tert-butylphenol, 2-tert-butyl-4-methylphenol,2-tert-butyl-5-methylphenol, and 6-tert-butyl-3-methylphenol;alkoxyphenols such as p-methoxyphenol, m-methoxyphenol, p-ethoxyphenol,m-ethoxyphenol, p-propoxyphenol, and m-propoxyphenol; isopropenylphenolssuch as o-isopropenylphenol, p-isopropenylphenol,2-methyl-4-isopropenylphenol, and 2-ethyl-4-isopropenylphenol;polyhydroxyphenols such as 4,4′-dihydroxybiphenyl, bisphenol A,phenylphenol, resorcinol, hydroquinone, pyrogallol; α-naphthol,β-naphthol, dihydroxynaphthalene, and derivatives thereof.

The novolak phenolic resin is prepared using at least 50% by weight ofp-cresol as the phenol reactant. Although the novolak phenolic resin maybe a homopolymer using 100% by weight of p-cresol, that is, p-cresolnovolak phenolic resin, the novolak phenolic resin is preferably acopolymer prepared using 50 to 80% by weight, especially 55 to 70% byweight of p-cresol and the balance of another phenol as the phenolreactant because the copolymer resin has a higher average molecularweight.

Instead of the copolymer resin mentioned above, it is acceptable to usea mixture of at least 50% by weight, preferably 50 to 80% by weight, andmore preferably 55 to 70% by weight of a p-cresol homopolymer resin(i.e., p-cresol novolak resin) and the balance of another novolakphenolic resin which is a polymer using a phenol other than p-cresol ora copolymer using two or more phenols other than p-cresol.

Suitable other phenols are as illustrated above. Inter alia, m-cresol,o-cresol, phenol, 2-allylphenol, 2,3-xylenol, 2,5-xylenol, 3,5-xylenol,and 3,4-xylenol are suitable.

It is not recommended to use a novolak phenolic resin using p-cresol inan amount of less than 50% by weight of the entire phenol reactant, anda mixture containing less than 50% by weight of the p-cresol novolakresin. This is because a resist film formed therefrom experiences asubstantial thickness change before and after development, which willgive rise to problems in the subsequent steps such as etching andelectrolytic plating.

The other reactant from which the novolak phenolic resin is prepared isan aldehyde. Any well-known aldehydes may be used. Suitable aldehydesinclude formaldehyde, paraformaldehyde, chloroacetaldehyde,dichloroacetaldehyde, bromoacetaldehyde, trioxane, benzaldehyde,acetaldehyde, p-nitrobenzaldehyde, p-acetoxybenzaldehyde,p-acetylaminobenzaldehyde, hydroxybenzaldehyde, dihydroxybenzaldehyde,vanillin, ethylvanillin, glyoxal, acrolein, and methacrolein. Interalia, formaldehyde, hydroxybenzaldehyde, and chloroacetaldehyde arepreferred.

The molecular weight of the novolak phenolic resin (A) obtained fromcondensation of a phenol containing at least 50% by weight of p-cresoland an aldehyde, the p-cresol novolak resin (b-1) obtained fromcondensation of p-cresol and an aldehyde, and the other novolak phenolicresin (b-2) obtained from condensation of a phenol other than p-cresoland an aldehyde is not particularly limited. These resins preferablyhave a weight average molecular weight (Mw) of at least 1,500, morepreferably at least 3,000, even more preferably at least 5,000 asmeasured by gel permeation chromatography (GPC) versus polystyrenestandards, for effective conversion to a higher molecular weight.Although the upper limit of Mw is not critical, the Mw is preferably upto 30,000, more preferably up to 20,000.

According to the invention, the modified novolak phenolic resin (I) isobtained by reacting the novolak phenolic resin (II) which is thenovolak phenolic resin (A) or mixture (B) with a crosslinker (III).

Suitable crosslinkers include an amino condensate modified with formalinor formalin-alcohol, a phenol compound having on average at least twomethylol or alkoxymethylol groups in a molecule, and an epoxy compoundhaving on average at least two epoxy groups in a molecule, which may beused alone or in admixture of two or more.

Examples of the amino condensate modified with formalin orformalin-alcohol include melamine condensates modified with formalin orformalin-alcohol, and urea condensates modified with formalin orformalin-alcohol.

The melamine condensate modified with formalin or formalin-alcohol isprepared, for example, by modifying a melamine monomer with formalin ina standard way into a methylol form, and optionally further modifying itwith alcohol into an alkoxy form. There is obtained a modified melaminehaving the general formula (1). The alcohol used herein is preferablyselected from lower alcohols, for example, alcohols of 1 to 4 carbonatoms.

Herein R¹ may be the same or different and is a methylol group, analkoxymethyl group having alkoxy moiety of 1 to 4 carbon atoms, orhydrogen, with the proviso that at least one R¹ is a methylol oralkoxymethyl group.

Examples of R¹ include a methylol group, alkoxymethyl groups such asmethoxymethyl and ethoxmethyl, and hydrogen.

Examples of the modified melamine having formula (1) includetrimethoxymethylmonomethylolmelamine,dimethoxymethylmonomethylolmelamine, trimethylolmelamine,hexamethylolmelamine, and hexamethoxymethylolmelamine.

Then the modified melamine having formula (1) or an oligomer thereof(such as dimer or trimer) is subjected to addition condensationpolymerization with formaldehyde in a standard way until the desiredmolecular weight is reached. There is obtained a melamine condensatemodified with formalin or formalin-alcohol.

The urea condensate modified with formalin or formalin-alcohol isprepared, for example, by modifying a urea condensate having apreselected molecular weight with formalin in a standard way into amethylol form, and optionally further modifying it with alcohol into analkoxy form.

Examples of the urea condensate modified with formalin orformalin-alcohol include methoxymethylated urea condensates,ethoxymethylated urea condensates, and propoxymethylated ureacondensates.

The modified melamine condensates and the modified urea condensate maybe used alone or in admixture of two or more.

Suitable phenol compounds having on average at least two methylol oralkoxymethylol groups in a molecule include(2-hydroxy-5-methyl)-1,3-benzenedimethanol and2,2′,6,6′-tetramethoxymethylbisphenol A. These phenol compounds may beused alone or in admixture of two or more as the crosslinker.

Suitable epoxy compounds having on average at least two epoxy groups ina molecule include bisphenol epoxy resins such as bisphenol A epoxyresins and bisphenol F epoxy resins, novolak epoxy resins such as phenolnovolak epoxy resins and cresol novolak epoxy resins, triphenolalkaneepoxy resins and polymers thereof, biphenyl epoxy resins,dicyclopentadiene-modified phenol novolak epoxy resins, phenolaralkylepoxy resins, biphenylaralkyl epoxy resins, naphthalene ring-containingepoxy resins, glycidyl ester epoxy resins, alicyclic epoxy resins, andheterocyclic epoxy resins.

Of the foregoing crosslinkers, the modified melamine condensates andmodified urea condensates are preferred for ease of reaction control.

The crosslinker (III) may be used alone or in admixture of two or more.The crosslinker (III) is preferably used in an amount of 0.1 to 50parts, more preferably 0.5 to 30 parts by weight per 100 parts by weightof the novolak phenolic resin (II). Less than 0.1 pbw of the crosslinkermay fail to achieve the object of the invention whereas more than 50 pbwof the crosslinker may excessively promote the resin reaction, resultingin gelation.

In a preferred embodiment, the reaction of novolak phenolic resin (II)with crosslinker (III) in the presence of an acidic catalyst is carriedout in an organic solvent. Suitable organic solvents which can be usedherein include alcohols, glycols, ketones, ethers, lactones, andaromatics. Examples include alcohols such as methanol, ethanol, andpropanol; diols such as 1,3-butanediol, ethylene glycol, propyleneglycol, and diethylene glycol; ketones such as dimethyl ketone, methylethyl ketone, diethyl ketone, and methyl isobutyl ketone; ethers such as1,4-dioxane, tetrahydrofuran, cyclopentyl methyl ether, ethylene glycolmonomethyl ether, and ethylene glycol monoethyl ether; lactones such asγ-butyrolactone; and aromatic hydrocarbons such as toluene and xylene.The solvent is not limited to the foregoing examples as long as thenovolak phenolic resin containing at least 50% by weight of p-cresol andthe crosslinker are uniformly dissolved therein.

The organic solvent may be used alone or in admixture of two or more.The amount of the organic solvent used is not particularly limited aslong as the novolak phenolic resin (II) and the crosslinker (III) areuniformly dissolved therein. Specifically, the solvent is used in anamount of 0.5 to 100 times greater, preferably 0.8 to 50 times greater,more preferably 1.0 to 10 times greater than 100 parts by weight ofnovolak phenolic resin (II).

Suitable acidic catalysts include mineral acids such as hydrochloricacid, sulfuric acid and boric acid; and organic acids such as oxalicacid, acetic acid, benzenesulfonic acid, p-toluenesulfonic acid,xylenesulfonic acid, p-phenolsulfonic acid, methanesulfonic acid, andethanesulfonic acid, which may be used alone or in admixture of two ormore. Preferably organic sulfonic acids are used because of highsolubility in organic solvents and strong acidity.

The acidic catalyst is preferably used in an amount of 0.1 to 70 parts,more preferably 0.5 to 50 parts, and even more preferably 1.0 to 30parts by weight per 100 parts by weight of the novolak phenolic resin(II). The acidic catalyst may be initially dissolved in the organicsolvent at the same time as novolak phenolic resin (II) and crosslinker(III), or later added dropwise to a solution of novolak phenolic resin(II) and crosslinker (III) in the organic solvent. Preferably theaddition of the crosslinker is followed by the addition of the acidiccatalyst.

Independent of whether the acidic catalyst is initially mixed withnovolak phenolic resin (II) and crosslinker (III) or later addedthereto, the temperature of the solution during reaction is preferablyat least 10° C. If the acidic catalyst is added at a lower temperature,the reaction may be retarded. As the reaction temperature is higher, thereaction time becomes shorter. At an extremely high temperature,however, the reaction may become difficult to control. The reaction ispreferably performed at a temperature of up to 100° C., more preferably10 to 80° C., though not essential.

At the end of reaction, a basic substance such as pyridine ortriethylamine is preferably added to control pH and quench the reaction,ensuring that the residual or unchanged acidic catalyst is deactivated.If base neutralization is difficult or inadequate, the acidic catalystmay be removed from the reaction solution by repeating several times thesteps of adding deionized water to the reaction solution, stirring,stationary holding for layer separation, and removing the water layerout of the system. In the latter case, a basic substance such aspyridine may be dissolved in deionized water whereby neutralization andwater washing are simultaneously performed. If it is unnecessary toquench the reaction completely, this catalyst removal step may beomitted. Further, if necessary, solvent exchange may be carried out, orthe solvent may be removed by evaporation to dryness.

Through the foregoing steps, there is obtained a modified novolakphenolic resin (I) in which the hydrogen atom of some phenolic hydroxylgroups on novolak phenolic resin (II) is replaced by crosslinker (III).

The crosslinked/modified resin (I) has a higher molecular weight,specifically a weight average molecular weight (Mw) of at least 2,000,preferably at least 4,000, and more preferably at least 6,000 and up to30,000, as measured by GPC versus polystyrene standards. The increase ofmolecular weight is not particularly limited as long as it is positive.Preferably the molecular weight of the modified resin (I) is at least500, more preferably at least 1,500 higher than the molecular weight ofthe starting novolak phenolic resin (II). The upper limit of themolecular weight increase is not particularly limited although themolecular weight increase is typically up to 20,000.

The modified novolak phenolic resin (I) thus obtained may be formulatedas a resist composition by combining with well-known photosensitiveagents, photoacid generators, basic compounds, crosslinkers, dissolutionaccelerators, dissolution inhibitors, dyes, surfactants, and the like.

The modified novolak phenolic resin (I) may be used as a negativeworking resist composition. In this case, a resist solution may beprepared by dissolving 100 parts by weight of the modified novolakphenolic resin (I), 0.05 to 50 parts by weight of a photoacid generator,and 1 to 50 parts by weight of a crosslinker in a resist solvent, andoptionally adding any well-known additives such as basic compound,surfactant, dye and dissolution accelerator. The resulting solution maybe used as a negative resist solution directly or as a negative resistfilm after coating the resist solution and drying the coating to form auniform film.

The modified novolak phenolic resin (I) may also be used as a positiveworking resist composition. The positive resist composition is preferredand may include the following embodiments.

(1) One embodiment is a composition comprising the modified novolakphenolic resin (I), a phenol compound in which some or all phenolichydroxyl groups are substituted with 1,2-naphthoquinonediazidosulfonicacid, and optionally, an unmodified novolak phenolic resin or anunmodified novolak phenolic resin in which some or all phenolic hydroxylgroups are substituted with 1,2-naphthoquinonediazidosulfonic acid.

(2) Another embodiment is a composition comprising the modified novolakphenolic resin (I) in which the hydrogen atom of some or all phenolichydroxyl groups is substituted by a 1,2-naphthoquinonediazidosulfonylgroup, and optionally, a phenol compound in which some or all phenolichydroxyl groups are substituted with 1,2-naphthoquinonediazidosulfonicacid and/or an unmodified novolak phenolic resin and an unmodifiednovolak phenolic resin in which some or all phenolic hydroxyl groups aresubstituted with 1,2-naphthoquinonediazidosulfonyl groups.

A resist solution may be prepared by optionally adding any well-knownadditives such as surfactant, photoacid generator, dye, crosslinker anddissolution accelerator to composition (1) or (2), and dissolving themin any well-known organic solvent. The resulting solution may be used asa positive resist solution directly or as a positive resist film aftercoating the resist solution and drying the coating to form a uniformfilm.

In either of the compositions, the naphthoquinonediazidosulfonic acidcompound may be used in an amount of 0.1 to 40%, preferably 0.5 to 30%by weight of the modified novolak phenolic resin.

When the naphthoquinonediazidosulfonic acid compound is incorporatedinto the novolak phenolic resin, 0.1 to 40 parts by weight of anaphthoquinonediazidosulfonic acid halide, typicallynaphthoquinonediazidosulfonic acid chloride is added to 100 parts byweight of the novolak phenolic resin. Substitution is effected in thepresence of a base such as triethylamine at a temperature in the rangeof 5 to 60° C., whereby the acid is incorporated into the resin.

(3) A further embodiment relates to the modified novolak phenolic resinin which some phenolic hydroxyl groups are substituted with well-knownacid labile groups. For example, phenolic hydroxyl groups are reactedwith a halogenated alkyl ether compound in the presence of a base,thereby yielding a modified novolak phenolic resin in which phenolichydroxyl groups are partially protected with alkoxyalkyl groups (i.e.,the hydrogen atom of phenolic hydroxyl groups is substituted by analkoxyalkyl group). The reaction solvent is preferably selected fromaprotic polar solvents such as acetonitrile, acetone, dimethylformamide,dimethylacetamide, tetrahydrofuran, and dimethyl sulfoxide, which may beused alone or in admixture. Suitable bases include triethylamine,pyridine, diisopropylamine, and potassium carbonate. The base ispreferably used in an amount of at least 10 mol % based on the overallmolar amount of phenolic hydroxyl groups. The reaction temperature is−50° C. to 100° C., preferably 0° C. to 60° C., and the reaction time is0.5 to 100 hours, preferably 1 to 20 hours.

The mode of introduction of an acid labile group is not limited to theabove embodiment. Any of well-known modes of introducing an acid labilegroup into a phenolic hydroxyl group may be used.

The thus obtained modified novolak phenolic resin having an acid labilegroup partially introduced therein is dissolved in an organic solventalong with a photoacid generator, obtaining a positive resist solution.If necessary, any of well-known additives such as basic compound,surfactant, dye, crosslinker, dissolution accelerator and dissolutioninhibitor may be added. The resulting solution may be used as a positiveresist solution directly or as a positive resist film after coating theresist solution and drying the coating to form a uniform film. Informing a resist film, a support film is typically used. The supportfilm may be a single film or a multilayer film consisting of a pluralityof laminated polymer layers. The support film may be made of syntheticresins such as polyethylene, polypropylene, polycarbonate, andpolyethylene terephthalate. Inter alia, polyethylene terephthalate ispreferred for appropriate flexibility, mechanical strength and heatresistance. The film may be pre-treated such as by corona treatment orcoating of parting agent.

Using the positive resist composition, a resist pattern may be formed byany standard methods. The resist composition is applied onto a substratein a suitable coating weight by a suitable coating technique. Thesubstrate is selected from substrates of Si, SiO₂, SiN, SiON, TiN, WSi,BPSG, and SOG, metal substrates of Au, Ti, W, Cu, Ni—Fe, Ta, Zn, Co, andPb, and organic antireflective coatings. Coating techniques include spincoating, roll coating, flow coating, dip coating, spray coating, anddoctor coating. The coating is pre-baked on a hot plate at 60 to 150° C.for 1 to 10 minutes, preferably at 80 to 120° C. for 1 to 5 minutes toform a resist film of desired thickness. The resist film is exposedthrough a mask having a desired pattern to radiation such as UV, deep-UVor electron beam, preferably radiation having a wavelength of 200 to 500nm. Exposure is preferably made in a dose of 1 to 1,000 mJ/cm², morepreferably 10 to 800 mJ/cm². The resist film is post-exposure baked(PEB) at 60 to 150° C. for 1 to 5 minutes, preferably at 80 to 120° C.for 1 to 3 minutes.

Then the resist film is developed in a developer in the form of aqueousalkaline solution, typically of tetramethylammonium hydroxide (TMAH),for 0.1 to 60 minutes, preferably 0.5 to 10 minutes, by a standardtechnique such as dip, puddle or spray development. The desired patternis formed on the substrate. Finally, a metal film is formed on thepattern by a suitable metallization technique such as sputtering orevaporation. The resist pattern is stripped together with the metal filmlying thereon, leaving a metal wiring on the substrate.

EXAMPLE

Examples of the invention are given below by way of illustration and notby way of limitation. Mw is a weight average molecular weight asmeasured by gel permeation chromatography (GPC) versus polystyrenestandards. Tg is glass transition temperature.

Example 1

To a solution of 300 g of a cresol novolak type phenolic resin having aMw of 7,000 (Tg 105° C., using a phenol reactant consisting of 55 wt %of p-cresol and 45 wt % of m-cresol and an aldehyde reactant offormaldehyde) and 3.0 g of a crosslinker CL-1 having formula (2)(Nikalac® MW-390, Sanwa Chemical Co., Ltd.) in 600 g of cyclopentylmethyl ether, a solution of 6.0 g of p-toluenesulfonic acid in 60 g ofcyclopentyl methyl ether was added dropwise at room temperature over 10minutes. The solution was stirred at 25° C. for a further 2 hours(maturing). The resulting solution was analyzed by GPC, finding amodified resin having a Mw of 8,200.

Examples 2 to 9

The reaction of novolak phenolic resin with crosslinker was carried outas in Example 1 aside from using the components shown in Table 1. Therewere obtained modified resins with Mw shown in Table 1.

TABLE 1 Example 2 3 4 5 6 7 8 9 Novolak Mw: 3,000 Mw: 7,000 Mw: 3,000Mw: 7,000 Mw: 7,000 Mw: 7,000 Mw: 7,000 Mw: 3,000 phenolic (100 g) (100g) (100 g) (100 g) (100 g) (100 g) (100 g) (100 g) resin Organic acetonetetra- tetra- propylene cyclopentyl tetra- tetra- cyclopentyl solvent(150 g) hydrofuran hydrofuran glycol methyl ether hydrofuran hydrofuranmethyl ether (500 g) (300 g) (500 g) (500 g) (500 g) (500 g) (150 g)Crosslinker Nikalac Nikalac Nikalac Nikalac EXA-850 Nikalac NikalacNikalac MX-270 MW-390 MX-270 MW-390 CRP MW-390 MW-390 MW-390 (10 g) (10g) (30 g) (15 g) (10 g) (10 g) (10 g) (10 g) Acidic oxalic trifluoro-p-toluene- hydro- methane- trifluoro- trifluoro- p-toluene- catalystacid methane- sulfonic chloric sulfonic methane- methane- sulfonic (10g) sulfonic acid acid acid sulfonic sulfonic acid acid (40 g) (5 g) (3g) acid acid (10 g) (5 g) (2 g) (8 g) Reaction 25° C./ 25° C./ 60° C./25° C./ 25° C./ 25° C./ 25° C./ 3° C./ conditions 2 hr 4 hr 4 hr 4 hr0.5 hr 4 hr 4 hr 2 hr maturing maturing maturing maturing maturingmaturing maturing maturing during after and after dropwise dropwiseaddition addition Final Mw 3,600 8,600 3,800 7,800 10,500 7,600 9,8003,300 *In Example 9, low temperature (3° C.) maturing was followed byquenching with triethylamine.

The novolak phenolic resin with Mw 7,000 was the same as in Example 1.The novolak phenolic resin with Mw 3,000 was a resin (Tg 90° C.) using aphenol reactant consisting of 60 wt % of p-cresol, 35 wt % of m-cresoland 5 wt % of 2,5-xylenol, and the same aldehyde reactant as above.

Tg of modified resins was measured by a differential scanningcalorimeter by Mettler-Toledo International Inc.

TABLE 2 Example 1 2 3 4 5 6 7 8 9 Tg (° C.) 130 100 130 105 125 138 125135 93

Comparative Examples are reported in Table 3.

TABLE 3 Comparative Example 1 2 3 4 5 Novolak Mw: 3,000 Mw: 7,000 Mw:3,000 Mw: 7,000 Mw: 1,300 phenolic (100 g) (100 g) (100 g) (100 g) (100g) resin Organic cyclopentyl tetra- tetra- propylene tetra- solventmethyl ether hydrofuran hydrofuran glycol hydrofuran (150 g) (500 g)(300 g) (200 g) (200 g) Crosslinker — Nikalac Nikalac — Nikalac MW-390MX-270 MW-390 (10 g) (30 g) (10 g) Acidic p-toluene- — — hydrochloricmethane- catalyst sulfonic acid acid sulfonic acid (10 g) (5 g) (5 g)Reaction 40° C./4 hr 40° C./4 hr 60° C./4 hr 25° C./4 hr 40° C./4 hrconditions maturing maturing maturing maturing maturing after afterduring after dropwise dropwise and after dropwise addition additiondropwise addition addition Final Mw 3,000 7,000 3,000 7,000 1,500

The cresol novolak type phenolic resin used in Comparative Example 5 wasa resin (Tg 50° C.) using a phenol reactant consisting of 40 wt % ofp-cresol and 60 wt % of m-cresol. After the completion of reaction, theresin had Tg of 55° C.

By adding 2.76 g of pyridine to the resin solution of Example 1 andvacuum drying, 290 g of a solid was recovered, which was used in thefollowing Application Example.

Application Example 1

A resist solution was prepared by dissolving 50 g of the solid, 3 g ofcrosslinker CL-1 (above), 0.3 g of photoacid generator PAG-1 havingformula (3), and 0.1 g of surfactant X-70-093 (Shin-Etsu Chemical Co.,Ltd.) in 100 g of propylene glycol monomethyl ether acetate (PGMEA), andfiltering through a membrane filter with a pore size of 0.2 μm.

The resist solution was applied onto a silicon wafer by spin coating andprebaked on a hot plate at 100° C. for 120 seconds, forming a resistfilm of 6 μm thick.

The resist film was exposed to i-line through a mask by means of a maskaligner MA-8 (SUSS MicroTec AG), baked (PEB) on a hot plate at 100° C.for 120 seconds, and developed in a developer, i.e., a 2.38 wt % TMAHaqueous solution for 200 seconds. This was followed by deionized waterrinsing for 30 seconds and spin drying. The resulting substrate wasobserved under a field emission scanning electron microscope S-4700(Hitachi Hitechnologies Ltd.). The substrate was found to bear aniterating 10-μm line-and-space pattern, with no problems detected.Before and after development, the thickness of the resist film wasmeasured at five points (n=5) by an optical interferometry filmthickness meter, finding a film thickness loss of less than 20 nm at allpoints.

Notably, a resist film is regarded as experiencing a film thickness losswhen the film thickness after development is less than 90% of the resistfilm thickness after coating and prebake.

Next, with stirring, 95 g of the solid obtained by vacuum drying theresin solution of Example 1 and 7.1 g of1,2-naphthoquinone-2-diazido-5-sulfonyl chloride were dissolved in 300 gof 1,4-dioxane. To the solution at room temperature, 2.8 g oftriethylamine was added dropwise. At the end of addition, stirring wascontinued for one hour. The reaction solution was poured into a largevolume of 0.1N hydrochloric acid aqueous solution, whereupon aprecipitated resin was recovered. The resin was vacuum dried, obtaining100 g of the intended photosensitive resin, a partially1,2-naphthoquinone-2-diazido-5-sulfonylated modified novolak phenolicresin. Next, a resist solution was prepared by dissolving 100 g of thepartially 1,2-naphthoquinone-2-diazido-5-sulfonylated modified novolakphenolic resin and 0.1 g of surfactant X-70-093 in 200 g of PGMEA andfiltering through a membrane filter with a pore size of 0.2 μm. Theresist solution was applied onto a silicon wafer by spin coating andprebaked on a hot plate at 100° C. for 120 seconds, forming a resistfilm of 6 μm thick. The resist film was exposed by means of mask alignerMA-8 (SUSS MicroTec AG), and developed in a 2.38 wt % TMAH aqueoussolution for 200 seconds. This was followed by deionized water rinsingfor 30 seconds and spin drying. The resulting substrate was observedunder SEM S-4700. The substrate was found to bear an iterating 10-μmline-and-space pattern, with no problems detected. The substrate wasfurther heated at 120° C. for 5 minutes. The iterating 10-μmline-and-space pattern as heated was observed under an electronmicroscope, finding no changes of shape and size before and afterheating.

In a further run, a resist solution was prepared by dissolving 50 g ofthe solid obtained by vacuum drying the resin solution of Example 1, 15g of photoacid generator PAC-1 having formula (4) and 0.1 g ofsurfactant X-70-093 in 100 g of PGMEA and filtering through a membranefilter with a pore size of 0.2 μm. The resist solution was applied ontoa silicon wafer by spin coating and prebaked on a hot plate at 100° C.for 120 seconds, forming a resist film of 6 μm thick. The resist filmwas exposed by means of mask aligner MA-8 (SUSS MicroTec AG) anddeveloped in a 2.38 wt % TMAH aqueous solution for 200 seconds. This wasfollowed by deionized water rinsing for 30 seconds and spin drying. Theresulting substrate was observed under SEM S-4700. The substrate wasfound to bear an iterating 10-μm line-and-space pattern, with noproblems detected.

A solution was prepared by dissolving 100 g of the solid obtained byvacuum drying the resin solution of Example 1 and 2 g of methanesulfonicacid in a mixture of 200 g of cyclopentyl methyl ether and 200 g oftetrahydrofuran. The solution was cooled at 3° C. in an ice bathwhereupon 20 g of ethyl vinyl ether was added dropwise. At the end ofaddition, 6.3 g of triethylamine was added to the solution which waskept in the ice bath for 10 minutes. With the ice bath removed at thispoint, the solution warmed up to 25° C. after 90 minutes. The solutionwas repeatedly washed with 0.1 wt % acetic acid aqueous solution untilpH<4. On subsequent vacuum drying, 116 g of a solid was recovered.

A resist solution was prepared by dissolving 100 g of the thus obtainedresin, 1.5 g of photoacid generator PAG-1, 0.1 g of triethanolamine and0.1 g of surfactant X-70-093 in 200 g of PGMEA and filtering through amembrane filter with a pore size of 0.2 μm. The resist solution wasapplied onto a silicon wafer by spin coating and prebaked on a hot plateat 120° C. for 120 seconds, forming a resist film of 6 μm thick. Theresist film was exposed to i-line by means of mask aligner MA-8 (SUSSMicroTec AG), baked (PEB) on a hot plate at 100° C. for 120 seconds, anddeveloped in a 2.38 wt % TMAH aqueous solution for 200 seconds. This wasfollowed by deionized water rinsing for 30 seconds and spin drying. Theresulting substrate was observed under SEM S-4700. The substrate wasfound to bear an iterating 10-μm line-and-space pattern, with noproblems detected.

In a further run, with stirring, 95 g of the solid obtained by vacuumdrying the resin solution of Comparative Example 2 and 7.1 g of1,2-naphthoquinone-2-diazido-5-sulfonyl chloride were dissolved in 300 gof 1,4-dioxane. To the solution at room temperature, 2.8 g oftriethylamine was added dropwise. At the end of addition, stirring wascontinued for one hour. The reaction solution was poured into a largevolume of 0.1N hydrochloric acid aqueous solution, whereupon aprecipitated resin was recovered. The resin was vacuum dried, obtaining100 g of the intended photosensitive resin, a partially1,2-naphthoquinone-2-diazido-5-sulfonylated modified novolak phenolicresin. Next, a resist solution was prepared by dissolving 100 g of thepartially 1,2-naphthoquinone-2-diazido-5-sulfonylated modified novolakphenolic resin and 0.1 g of surfactant X-70-093 in 200 g of PGMEA andfiltering through a membrane filter with a pore size of 0.2 μm. Theresist solution was applied onto a silicon wafer by spin coating andprebaked on a hot plate at 100° C. for 120 seconds, forming a resistfilm of 6 μm thick. The resist film was exposed to i-line by means ofmask aligner MA-8 (SUSS MicroTec

AG), and developed in a 2.38 wt % TMAH aqueous solution for 200 seconds.This was followed by deionized water rinsing for 30 seconds and spindrying. The resulting substrate was observed under SEM S-4700. Thesubstrate was found to bear an iterating 10-μm line-and-space pattern,with no problems detected. The substrate was further heated at 120° C.for 5 minutes. The iterating 10-μm line-and-space pattern as heated wasobserved under an electron microscope, finding that the pattern profileas heated was less rectangular and the width of lines was increased nearthe substrate. It was demonstrated that the pattern deformed by thermalflow since the resin had low heat resistance.

In a final run, with stirring, 95 g of the solid obtained by vacuumdrying the resin solution of Comparative Example 5 and 10.1 g of1,2-naphthoquinone-2-diazido-5-sulfonyl chloride were dissolved in 300 gof 1,4-dioxane. To the solution at room temperature, 2.8 g oftriethylamine was added dropwise. At the end of addition, stirring wascontinued for one hour. The reaction solution was poured into a largevolume of 0.1N hydrochloric acid aqueous solution, whereupon aprecipitated resin was recovered. The resin was vacuum dried, obtaining100 g of the intended photosensitive resin, a partially1,2-naphthoquinone-2-diazido-5-sulfonylated modified novolak phenolicresin. Next, a resist solution was prepared by dissolving 100 g of thepartially 1,2-naphthoquinone-2-diazido-5-sulfonylated modified novolakphenolic resin and 0.1 g of surfactant X-70-093 in 200 g of PGMEA andfiltering through a membrane filter with a pore size of 0.2 μm. Theresist solution was applied onto a silicon wafer by spin coating andprebaked on a hot plate at 100° C. for 120 seconds, forming a resistfilm of 6 μm thick. The resist film was exposed to i-line by means ofmask aligner MA-8 (SUSS MicroTec AG), and developed in a 2.38 wt % TMAHaqueous solution for 200 seconds. This was followed by deionized waterrinsing for 30 seconds and spin drying. The thickness of the film afterdevelopment was measured to be 68% of the thickness of the prebakedfilm, which was practically unacceptable.

Japanese Patent Application Nos. 2011-233901 and 2012-176987 areincorporated herein by reference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A modified novolak phenolic resin obtained by reacting (A) a novolakphenolic resin obtained from condensation of a phenol containing atleast 50% by weight of p-cresol and an aldehyde, or (B) a mixture of(b-1) at least 50% by weight of a p-cresol novolak resin obtained fromcondensation of p-cresol and an aldehyde and (b-2) the balance ofanother novolak phenolic resin obtained from condensation of a phenolother than p-cresol and an aldehyde with a crosslinker.
 2. The modifiednovolak phenolic resin of claim 1 wherein the phenol from which thenovolak phenolic resin (A) is obtained consists of 50 to 80% by weightof p-cresol and the balance of another phenol.
 3. The modified novolakphenolic resin of claim 1 wherein the mixture (B) consists of 50 to 80%by weight of the p-cresol novolak resin (b-1) and the balance of theother novolak phenolic resin (b-2).
 4. The modified novolak phenolicresin of claim 1 wherein the novolak phenolic resin (A), the p-cresolnovolak resin (b-1), and the other novolak phenolic resin (b-2) eachhave a weight average molecular weight in the range of 1,500 to 10,000.5. The modified novolak phenolic resin of claim 1 wherein thecrosslinker is at least one member selected from the group consisting ofan amino condensate modified with formalin or formalin-alcohol, a phenolcompound having on average at least two methylol or alkoxymethylolgroups in a molecule, and an epoxy compound having on average at leasttwo epoxy groups in a molecule.
 6. The modified novolak phenolic resinof claim 5 wherein the crosslinker is a modified melamine condensate ormodified urea condensate.
 7. A method of preparing a modified novolakphenolic resin, comprising the steps of mixing (A) a novolak phenolicresin obtained from condensation of a phenol containing at least 50% byweight of p-cresol and an aldehyde, or (B) a mixture of (b-1) at least50% by weight of a p-cresol novolak resin obtained from condensation ofp-cresol and an aldehyde and (b-2) the balance of another novolakphenolic resin obtained from condensation of a phenol other thanp-cresol and an aldehyde with a crosslinker, simultaneously orsubsequently adding an acidic catalyst thereto, and effecting reaction.8. The method of claim 7 wherein the phenol from which the novolakphenolic resin (A) is obtained consists of 50 to 80% by weight ofp-cresol and the balance of another phenol.
 9. The method of claim 7wherein the mixture (B) consists of 50 to 80% by weight of the p-cresolnovolak resin (b-1) and the balance of the other novolak phenolic resin(b-2).
 10. The method of claim 7 wherein the novolak phenolic resin (A),the p-cresol novolak resin (b-1), and the other novolak phenolic resin(b-2) each have a weight average molecular weight in the range of 1,500to 10,000.
 11. The method of claim 7 wherein the crosslinker is at leastone member selected from the group consisting of an amino condensatemodified with formalin or formalin-alcohol, a phenol compound having onaverage at least two methylol or alkoxymethylol groups in a molecule,and an epoxy compound having on average at least two epoxy groups in amolecule.
 12. The method of claim 11 wherein the crosslinker is amodified melamine condensate or modified urea condensate.
 13. The methodof claim 7 wherein the acidic catalyst is at least one acid selectedfrom the group consisting of hydrochloric acid, sulfuric acid, boricacid, oxalic acid, acetic acid, benzenesulfonic acid, p-toluenesulfonicacid, xylenesulfonic acid, p-phenolsulfonic acid, methanesulfonic acid,and ethanesulfonic acid.
 14. The method of claim 7 wherein the acidiccatalyst is an organic sulfonic acid.
 15. The method of claim 7 whereinthe steps of simultaneously or subsequently adding an acidic catalystand effecting reaction include holding at a temperature of at least 10°C.
 16. A resist composition comprising the modified novolak phenolicresin of claim 1 as a base resin.
 17. The resist composition of claim 16which is a positive working composition.
 18. The resist composition ofclaim 16, further comprising a 1,2-naphthoquinonediazidosulfonic acidester.
 19. The resist composition of claim 18 wherein the1,2-naphthoquinonediazidosulfonic acid ester is an ester of some or allmodified phenolic hydroxyl groups on the modified novolak phenolic resinof claim 1 with 1,2-naphthoquinonediazidosulfonic acid.